Re: rows estimate in explain analyze for the BRIN index

> On 30 Dec 2015, at 18:38, Emre Hasegeli <emre@hasegeli.com> wrote:
>
>> which is much closer to the actual number of rows removed by the index
>> recheck + the one left.
>
> Is it better to be closer?  We are saying those are the "actual"
> values not the estimates.  If we cannot provide the actual rows, I
> think it is better to provide nothing.  Something closer to the
> reality would create more confusion.  Maybe, we just just return the
> number of blocks, and put somewhere a note about it.  The actual row
> count is already available on the upper part of the plan.

I don’t see how to solve this problem without changing explain analyze output to accommodate for “unknown” value. I
don’tthink “0” is a non-confusing representation of “unknown” for most people, and from the practical standpoint, a
“besteffort” estimate is better than 0 (i.e. I will be able to estimate how efficient BRIN index is for my tables in
termsof the number of tuples retrieved/thrown away) 

We might still reflect in the documentation that the BRIN index cannot produce the exact number of rows during the
bitmapscan and point people asking similar questions there.